To many hardwood flooring contractors, installing hardwood floors over concrete can be a bit of a puzzle. The following is a guide to help alleviate any concerns or worries about installing hardwood floors over concrete.
The secret behind a successful install over concrete is to focus on five key areas: Condition of slab, type of concrete, location of slab, flatness, and moisture.
Read on for in-depth details on these five key areas:
CONDITION OF THE CONCRETE SLAB
One of the easiest and most beneficial steps to take is to carefully examine the condition of the concrete slab. Take a look at the slab and note any cracks, spalling, or curling at expansion joints.
This is also the time to note any high or low spots in the concrete. Currently the National Wood Flooring Association (NWFA) recommends 1/8″ to 3/16″ in a 10FT radius. You may have to grind down the floor or use a self-leveling compound like Bostik SL-150 to even out the slab.
If you take the time to examine the concrete and set about to fix any potential problems, you will save yourself time and energy later on. After all, if concrete slab is in poor condition, your hardwood floors are more likely to fail.
MOISTURE CONTENT OF THE SLAB
Checking the moisture rating of a concrete slab is arguably the most important test to perform when assessing a concrete slab. Moisture is said to cause upwards of 90% of hardwood flooring failures.
When it comes to moisture and concrete slabs the two most recognized tests are calcium chloride and relative humidity. Click here to learn more about these two tests, and how they compare.
Understanding the moisture content of a concrete slab and taking the necessary steps to combat moisture will help immensely in successfully installing hardwood floors over concrete.
TYPE OF CONCRETE
We know that there are many different types of hardwood that perform differently. We pay attention to the type of wood and work with it in a way to get the best possible installation. The same is true with concrete.
There will be information (provided by architects, builders, and engineers) that gives specific information about the concrete slab in question. This information will include figures like PSI, CSP (Concrete Surface Profile), and Flatness (FL, FF, & F-min). Sometimes you’ll find the concrete is actually a lightweight concrete. In that particular scenario, you’ll have to take additional steps before installing the hardwood floors. If you didn’t know a concrete slab was lightweight concrete, you might have incorrectly installed the hardwood floor.
Gathering the information on the concrete will aid you during the install. You’ll be on the path to correctly install the hardwood floor on the specified concrete.
LOCATION OF SLAB
This step might be the easiest to determine. However, there are still things you need to take note of. When determining the location of the slab what you’re looking for is whether the slab is at, above, or below grade.
No matter where the concrete slab is located, you must know how to deal with it. Slabs above grade have less moisture than below grade slabs. However, slabs above grade might also have a weight limit. For example in a condominium you might not be able to put down leveling compound because of the weight it would add to the concrete slab. In a below grade install there is more moisture and usually that means adding moisture/vapor barriers.
In most scenarios when a concrete slab is poured, the concrete is poured and screeded into a pre-made form. The professionals pouring the concrete will surely make the slab seems level and flat, but that does not necessarily mean the concrete is flat enough to install hardwood floors.
Using one of many different tools — ten foot straight edge, mason’s string, or a laser — can determine the true flatness of a concrete slab. Typically a floor is flat when it is 3/16″ over a 10′ radius or 1/8″ over a 6′ radius.
If the concrete slab is not level then you either need to use leveling compound to fill in low spots or grind away high spots. Click here to learn more about the Bostik SL-150 self-leveling compound.
Or, if the situation requires it, installing a subfloor over the concrete slab might be the best option.
In any scenario, making sure the concrete is flat and level enough to install hardwood floors over is paramount.
Hardwood floors are installed over concrete by wood flooring mechanics all the time. When you are knowledgeable about how concrete works and how it interacts with hardwood flooring, you will be prepared to install a beautiful floor over concrete.
If you’d like to learn more about preparing a concrete slab for hardwood floors, call us at (800) 787-1786 or email info [at] cityfloorsupply.com